I have a 2000 chevy malibu ls. at first..we just thought that the battery needed charged..so we bought a new battery.. then it still wouldnt start..so we bought a new starter and replaced that..and it still wont start.when i turn the key everything else does what its supposed to but it just wont roll over and start. i have spark and everything. i just dont know what else it could be. can you help me please?

DO you hear a clicking when you turn the key to start? Solenoid relay make a click sound when energized.
If the solenoid is not mounted on the starter motor then it will be near the battery
The key switch sends power to the solenoid. the solenid is a big remote controlled switch,and it takes power from the battery to the starter motor

Here's a few things to check;
If the battery is good and has a full charge but engine is not turning over at all, check the leads (cables) at the starter is it getting power down to the starter, if not, could be bad connection, bad wires or the ignition switch itself.

If it is getting power at the starter, it could be the starter needs to be replaced. To check, turn the ignition (key) to on and cross the terminals at the starter with a screw driver. You'll see one cable from the starter go to ground (a bolt on the engine) and the other will go direct to the battery, they are the terminals you need to cross with the screw driver. While you're down ther check the terminals on the starter and the ground that goes to the engine, make sure they are making a connection, no corrosion around the terminals.

make sure battery connections are clean and tight. hope this helps.